In the exhaust system of an engine, for example in the case of a 4-cylinder engine, there is an exhaust manifold that brings the four branches corresponding to each cylinder into one. The exhaust manifold comprises: a head flange section that has an opening corresponding to the exhaust port of each cylinder; four branches that are connected to each of the openings on the flange section; and a collection section that collects the downstream side of all of the branches into one and directs them to the catalyst in order to clean the exhaust gas (for example, refer to Document 1).
Also, as disclosed in aforementioned Document 1, in some recent exhaust manifolds there is an open/close valve that switches between either closing or opening the exhaust pathway of the collection section. In this kind of exhaust manifold there is a bypass path between the upstream end section of each branch and the middle section of a main catalyst (downstream catalyst) that is located in the open/close valve and downstream of that open/close valve, and there is a pre-catalyst (upstream (front) catalyst) located inside that bypass path.
Also, during a cold startup, the open/close valve is in the closed state, and exhaust gas flows from the bypass path, through the pre-catalyst and then to the main catalyst. In this way, the pre-catalyst reaches a suitable activation temperature earlier than the main catalyst, and thus purification of the exhaust gas can be performed effectively. On the other hand, as the main catalyst rises to a suitable catalyst activation temperature, the open/close valve switches to the open state, and the exhaust gas is switched-over such that it flows directly to the main catalyst. In other words, when the exhaust gas is discharged by way of the bypass path (via pre-catalyst), the exhaust resistance is large and affects the engine output. Therefore, after the main catalyst has risen to a suitable activation temperature, the open/close value opens and switches-over the flow path of the exhaust gas to the main path (main catalyst) that has a low exhaust resistance.